Geology of Kilbourne Hole

Kilbourne Hole by Lee Stockman (Vice President for Programs)

The February Field Trip visited one of New Mexico’s more unusual geological features. Most geologic formations developed over eons. Kilbourne Hole was not one of these, but a spectacular event that was sudden. It has been described as “One of the worlds most perfect example of a maar.” A maar is a volcanic event which occurs when magma comes into contact with water below the surface. About 28,000 years ago when the weight of the overburden could not hold in the pressure generated by the super heated water a violent explosion sent the overburden flying.

The hole left in the ground had hardly any crater walls above the surrounding land. Those who were on the field trip may have noticed the hard light colored material deposited in layers on the inside of the crater where we climbed down to collect. These layers are deposits of the ash that was expelled at the time of the explosion. An unusual characteristic of the magma that produced the explosion is that it contained peridot crystal pockets which came up with the magma from the Mantle originating some 7 miles deep in the earth. Today when we walk along the walls of Kilbourne Hole we find the peridot in the cooled pockets with a coating of the magma on the outside.

The garnet granules in with the Peridot have been dated by the uranium lead method at 1.375 billion years so it had formed long before the explosion that brought it to the surface. What the Crater looked like just after the explosion, we will never know but it must have been deep. Other maars in New Mexico had craters almost a mile deep. But during the ensuing years alluvial deposits have buried the bottom of the crater and today we see a playa and even evidence of human occupation in the form of a foundation for a building.